Increasingly, today's vehicles are equipped with assistance and safety systems, such as e.g. navigation systems, ACC-systems (Adaptive Cruise Control), lane assistance systems, traffic sign recognition, etc. The function of many of these systems requires the determination of a distance to an object in the environment of the vehicle, for example of a preceding vehicle. Here, known methods such as ultrasonic, infrared, laser, radar, lidar, etc. are applied, wherein usually a coded signal is transmitted and the distance to the object is calculated on the basis of the running time of the reflected signal.
A generic method is described in EP 0 911 645 B1, wherein by means of an optical apparatus the distance and/or speed of an object are measured by means of a polarization-modulated transmission light beam. This optical apparatus comprises a laser diode for generating a longitudinal-polarized transmission light beam, the polarization plane thereof changing by means of a polarization modulator between a first polarization state and a second polarization state in accordance with a binary control signal. The transmission light beam backscattered at the object as reception light beam is converted by a polarization detector into an amplitude-modulated light beam, to generate therefrom by means of a detector an electrical signal, which is compared with the binary control signal controlling the polarization modulator. From the phase shift between these two signals the distance and/or the speed of the object are determined.